1. Field of the Invention
The present invention relates to information storage media possessing optical servo tracks and an apparatus and method for inscribing tracks on the media and more particularly to magnetic disk media in which the optical servo tracks comprise a plurality of concentric grooves inscribed in the surface of the disk, e.g. by a laser, while the disk rotates at high speeds.
2. Description of the Prior Art
The track density of magnetic storage disks for conventional floppy disk drives is approximately forty-eight to one hundred thirty-five tracks per inch (TPI). In contrast, optical disk drives are capable of achieving track densities in excess of 15,000 TPI. These higher track densitites are achieved through the use of closed loop optical servos that allow the read/write head to follow data track eccentricities caused by defects in the medium and by disturbances from outside forces. In rigid type magnetic disk drives, track densities of up to 1500 TPI are presently used. These drives commonly have multiple disks in which both sides are used for data. To achieve the high track density a dedicated surface of one of the disks is used for magnetic track servo information. This surface is then not usable for data storage. Thus, the total capacity of the drive is lessened. The tracking servo information is also capable of being accidentally erased causing loss of access to all data.
Various techniques have been reported for using optical means for acquiring track following servo information contained on a magnetic recording medium. For example, Ahn, et al in U.S. Pat. No. 4,633,451, issued on Dec. 30, 1986, for "Optical Servo For Magnetic Disks", disclose the use of a laser diode to read track following servo information in the form of a plurality of spots contained in an optical layer positioned above a magnetic recording layer.
DiStefano, et al, in U.S. Pat. No. 4,570,191, issued on Feb. 11, 1986, for "Optical Sensor for Servo Position Control", disclose a servo sensor comprising a light source and a light detector, axially aligned and contained on a single semiconductor chip.
M. Johnson, in U.S. Pat. No. 4,558,383, issued on Dec. 10, 1985, for "Information Storage Disk Transducer Position Control System Using a Prerecorded Servo Pattern Requiring No Alignment With the Storage Disk", discloses a servo apparatus having a sensor for detecting a pattern of spots on a surface of an information storage medium. The spots comprise a dense array of substantially translation invariant marks and separate information recording tracks are detected by measuring the rate at which the spots are detected by the sensor.
J. Cocke, et al, in U.S. Pat. No. 4,587,579, issued on May 6, 1986, for "System for Position Detection on a Rotating Disk", disclose a servo control system comprising a detector for reading a plurality of spiral radial-position-encoding patterns on a medium.
A. S. Hoagland in "Optical Servo of Magnetic Recording", IBM Technical Disclosure Bulletin, Vol. 20(10), page 4108 (March 1978), suggests a system for achieving optical servo control where a flexible disk medium includes a plurality of optical servo tracks positioned underneath a magnetic layer.
D. A. Thompson, et al in "Embossed Servo Techniques For Floppy Disks", IERE Conference Proceedings, No. 43, p. 321 (July 1979), suggests the use of embossed marks on flexible magnetic media for obtaining non-magnetic optical or capacitive servo information.
N. Koshino and S. Ogawa in "Optical Method of the Head Positioning in Magnetic Disk Systems", preprint from IEEE Transactions on Magnetics (1980), disclose an optical head for achieving track following servo control which is mounted on the head arm and which includes an LED light source and three optical fibers for delivering light to a medium. The medium comprises a plurality of circular optical tracks, dyed black, and located underneath a magnetic film.
Related work has occurred in the laser video disk area, from which optical disks for digital data storage and the audio laser disk (CD) have evolved. Fundamentally, the optical servo information is inscribed and used in the same way for all these disks. A mastering machine is used to format optical information onto a master disk. The master is then replicated to form the actual disk used by the customer. A laser and associated optics are used to acquire the mastered servo information as well as read data from the disk. The data can be inscribed during the mastering process as with the video and audio disks or it can be written by the read/write laser as in disks for digital information storage.
K. D. Broadbent in "A Review of the MCA Disco-Vision System", Journal of the SMPTE (1974), describes the Laser Video mastering technique as well as the servo and read back methods. The mastering machine uses an argon laser to ablate pits in a metallic layer which was deposited on a glass plate. Disks are replicated from the master which contain servo information as well as the video data. A technique for deriving the servo information is described.
The information used for the tracking servo is contained in the laser disks within the features (grooves or pits) that are on the data layer of the disk. The feature causes the reflected light to be modulated in such a way that an error signal can be derived. By the features depth and width being on the order of the wavelength of the laser light, reflected light is modulated by diffraction effects. The effect of diffraction are most pronounced with coherent light as emitted by a laser.